Variable inductors have been employed in a variety of applications where it is desired to be able to modify a characteristic of a frequency response of an electronic circuit. Specific applications which employ a variable inductor include timing circuits, tuning circuits, and calibration circuits. In a tuning circuit comprising an inductor and a capacitor, the use of a variable inductor may be preferred over a variable capacitor.
A common form of adjustment of a variable inductor involves a variation of an effective permeability of a magnetic path. The effective permeability of the magnetic path can be varied mechanically by modifying a location of a magnetic core within a wound helical coil. The effective permeability of the magnetic path can also be varied in an electrical manner. Here, an operating flux density is varied within the core material to modify the relative permeability therein.
An example of an electrically controllable inductor is shown in U.S. Pat. No. 4,620,144 to Bolduc. This variable inductor comprises a single magnetic core about which a primary coil and a control coil are wound. A direct current is supplied to the control coil for varying the inductance in the primary coil.
Disadvantages which result from electrically changing the permeability of the magnetic core of an inductor include current waveform distortion, limited capability in high power applications, and a limited range of variation of the inductance. Other disadvantages include the effect of increased heating of the core of an inductor in which the permeability is changed by increased saturation of the core. Further, the introduction of harmonics, or noise, on to the line is exacerbated by changing core permeability.
Inductors can be employed in the application of power factor correction and harmonic distortion reduction in the transmission of electrical power. In terms of an electric power transmission system comprising a power source, a load, and a line conductor connecting the load to the power source, the power factor of the load is defined as the ratio of the active power delivered to, or absorbed by, the load to the apparent power at the load. In response to a resulting expense incurred by loads having a low power factor, the rate structure employed by an electric utility company is such that the billing rate is increased by means of a penalty factor whenever the power factor of a customer drops below a threshold. For example, many utilities require that the power factor of industrial customers be an least 0.90 in order to benefit from the minimum billing rate. One method of correcting the power factor of a three-phase line entails adding balanced three-phase capacitors in parallel with the line.
The use of variable inductors in controlling power factor and harmonics was limited by inherent power and size limitations and the problems imposed by harmonics induced by such circuits themselves. Harmonics may be introduced to the line and existing harmonics may be aggravated by merely using capacitors to balance power factor in electrical lines. Although capacitors do not produce harmonics themselves, they can create circuits which resonate at frequencies at or near existing harmonic levels. Harmonic suppression is best achieved through use of appropriate harmonic filter inductors wired in parallel with the capacitor network. Such inductors are typically pre-tuned to specific harmonic frequencies.